Heathkit aa-121

[maritimer]

Hello all,

I have been fortunate to have been given a Heathkit AA-121. It has never been touched and is in need of tubes for sure and I would guess caps. Can someone refer me to a great source for caps as well as replacement tubes? I have purchased tubes for guitar amps in the past but never for hi fi. Any recommendations would be most appreciated and I'd love to hear from other AA-121 owners.

[Eli Duttman]

Dude,

The same sort of tubes get used for both HIFI and guitar. They get handled differently. FWIW, an old post on this "board" described a HIFI amp as guitar amp wearing lipstick and nylons.

Maybe Tom Bavis has the schematic for the AA-121 in his extensive collection. I was able to track down the amp's tube complement. It's 2X matched pairs of EL34s and 2X 6AN8s. 6AN8s are NOS only, but reasonably plentiful. In current production, the SED (=C=) EL34 is a safe choice. The sound is not too far off that of revered and astronomically expensive Mullard made NOS.

Jim McShane is a good source for tubes and some of the parts needed in the overhaul.

The bias supply in a stock AA-121 uses a Selenium rectifier. Selenium rectifiers are ticking, toxic, time bombs. Replacement with a modern Silicon diode is mandatory.. You will have to compensate for the smaller forward drop in Silicon.

The 4X OEM Silicon rectifiers in the AA-121 are very noisy by current standards. 1 A./1000 PIV rated UF4007s are an inexpensive upgrade. If you think 1 A. rated diodes are inadequate, use 3 A./1000 PIV rated UF5408s.

Obviously, all electrolytic caps. have to be replaced. Changing coupling caps. rates to be a good idea too. 716P series orange drops and Panasonic ECQ-P(U) series parts are reasonably priced, while having film and foil construction.

The OEM Carbon composition resistors have drifted in value and gone noisy, with age. Replace grid leak resistors with low noise metal film parts. Grid stoppers should remain Carbon comp. Carbon film resistors are OK elsewhere, except in NFB loops. Use 1% tolerance metal film parts in NFB loops, to insure both precision and accuracy. Unless cracking or other damage is present, any wirewound resistors in the amp should not need replacement.

[astouffer]

You will have to compensate for the smaller forward drop in Silicon.

On amps with adjustable bias is this really an issue? What is the forward voltage drop of a typical selenium rectifier? Not trying to start an argument or anything I'm just curious as to how bad they really were.

[koolatron]

What is the forward voltage drop of a typical selenium rectifier?

You'd be surprised at how non-obvious this seems to be, out on the internets. Cursory googling revealed only that the forward voltage drop is larger, but didn't give any figures. I happened upon a 1950s patent for selenium rectifiers (http://www.freepatentsonline.com/3694908.html) that seems to indicate that the forward voltage drop of a selenium rectifier is about 0.1V per layer. Each layer is capable of holding off about 20V in reverse bias, and layers are stacked to arrive at a PIV rating that makes the rectifier useful for electronics applications.

Using those numbers, it seems like dividing the PIV rating of a selenium rectifier by about 200 ought to give you its forward voltage drop. I'm no authority, though.

--k

[danFrank]

Selinium also has resistance when it is conducting so there is voltage drop from that also. The more current pulled, the more the voltage drop. A selenium rectifier is somewhere between a silicon diode and a tube rectifier in that matter. Since these selenium rectifiers where used for the most part in bias supplies, just substitute a UF1007 or UF4007 diode and 50K pot in place of the old selenium rectifier. Adjust the pot until you have the voltage you are looking for. Power down the amp, take the pot out of the circuit and take the reading of it with a multimeter, then sub the closest value of fixed resistor in place of the pot in the circuit. Tweek resistor value if needed.
Hope this helps...
Daniel